Uniformity of film etching rate and uniformity of film thickness has always been an important issue in plasma etching. Wafer-to-wafer uniformity within a batch is no longer a major concern because batch reactors that treat numerous small wafers are being replaced with large, single-wafer etchers. A dominant concern that remains is etching process uniformity across a single wafer, especially as silicon wafer diameters are increasing. See, J. W. Coburn, "Summary Abstract: Diagnostics in Plasma Processing," J. Vac. Sci. Technol. A 4(3), 1830 (1986). State-of-the-art wafer processing is done on 200-mm diameter wafers. The next generation of wafers will probably be 250 and/or 300-mm diameter.
Process parameters include the uniformity of etching rate, and the absolute thickness of specific structures. Presently, rate uniformity is determined ex situ, and after the etching step, by optically measuring film thickness (for example, with a Nanospec) before and after a partial etch of known duration. See generally, A. G. Nagy, "Radial Etch Rate Nonuniformity in Reactive Ion Etching," J. Electrochem. Soc. 131(8), 1871 (1984) and Alan S. Kao & Harvey G. Stenger, Jr., "Analysis of Nonuniformities in the Plasma Etching of Silicon," J. Electrochem. Soc. 137(3), 954 (1990). This process is not suitable as a diagnostic for real-time process control, i.e., control of the etching process as it is being conducted.
A number of techniques exist to measure etching rate and/or film thickness in situ. Among these are laser interferometry, optical emission interferometry ("OEI") and ellipsometry. See generally Paul J. Marcoux & Pang Dow Foo, "Methods of End Point Detection for Plasma Etching," Solid State Technology, 24(4), 115 (1981)(laser interferometry) and Sternheim, W. van Gelder & A. W. Hartman, "A Laser Interferometer System to Monitor Dry Etching of Patterned Silicon," J. Electrochem. Soc. 130(3), 655 (1983); F. Heinrich, H.-P. Still, and H.-C. Scheer, "New and Simple Optical Method to in situ etch rate determination and endpoint detection," Appl. Phys. Lett. 55(14), 1474 (1989)(OEI); and David Angell & Gottlied S. Oehrlein, "Grazing Angle Optical Emission Interferometry for End-Point Detection," Appl. Phys. Lett. 58(3), 240 (1991); and Steven A. Henck, "In situ Real-Time Ellipsometry for Film Thickness Measurement and Control," J. Vac. Sci. Technol. A 10 (4), 934 (1992).
Laser interferometry and optical emission interferometry both analyze the interference of light reflected from a thin film, but they use different light sources. Laser interferometry uses a laser beam (typically a 632.8 nm Helium-Neon (HeNe), while optical emission interferometry uses etch reactor plasma optical emission as the light source. Ellipsometry measures the change in polarization of light upon reflection of the light from a surface.
FIG. 1 depicts a thin film 100 of thickness d and refractive index n.sub.2 on a wafer substrate 120 with refractive index n.sub.3. A pedestal 122 supports the wafer substrate 120. (In some cases, more than one thin film layers will make up the thin film layer 100. In such a case, the light is reflected at each interface of different indices of refraction. Further, the upper most layer may have a patterned photoresist layer coating it.) Light rays 102 at a wavelength of .lambda. travel through a first medium 130 with a refractive index n.sub.1 and strike the interface between the medium 130 and the thin film 100 at an angle .theta..sub.1 relative to the surface normal. Part 104 of the light rays 102 are refracted inside the film to an angle .theta..sub.2 to the surface normal. Another part 106 of the incident light ray is reflected from the interface between the first medium 130 and the film 100. Following the development of Hecht & Zajac (Eugene Hecht & Alfred Zajac, OPTICS (Addison Wesley, Reading Mass., 1979), p.295-297), the optical path length difference, .DELTA.l.sub.opt, between light 106 reflected from the interface between the first medium 130 and the film 100 on the one hand, and light 108 that has traveled through the film 100 and been reflected from the lower interface between the film 100 and the wafer substrate 120 on the other hand can be written as: ##EQU1##
Light rays 110, which are the result of multiple reflections within the film, are usually sufficiently low in intensity that they can be neglected in the analysis discussed below. Using Snell's Law, equation (1) can be written in terms of .theta. alone, and simplified to yield: EQU .DELTA.l.sub.opt =2n.sub.2 d cos (.theta..sub.2). (2)
A maximum in the intensity of the net reflected light resulting from interference between the two reflected light rays 106 and 108 will occur when the optical path difference .DELTA.l.sub.opt is an integral multiple of the wavelength .lambda. of the light. If n.sub.1 is assumed to be 1, then .DELTA.d.sub.film, the change in film thickness that has occurred between the occurrence of adjacent (in time) maxima or minima (known as "extrema"), is given by: ##EQU2##
For normal incidence, (i.e., .theta..sub.1 =.theta..sub.2 =0) Eq. (3) reduces to simply ##EQU3##
The term cos (.theta..sub.2) is a correction for non-normal incidence, which can be rewritten in terms of measurable parameters (i.e., .theta..sub.1 and n.sub.2) as: ##EQU4##
A practical known apparatus for laser interferometry is shown in FIG. 2. An incident beam "I" from HeNe laser 202 strikes the interface between the wafer 208 and the chamber environment 230 such as a plasma of the reactor chamber 214. The reflected beam "R" is directed through a bandpass filter 210 to a photodiode 212, where an interferometry signal is recorded as a function of time. The bandpass filter 210 prevents plasma emission (i.e. unwanted light) from entering the photodiode 212, while allowing the reflected laser beam R to strike the photodiode 212.
Similarly, a known optical emission interferometry apparatus is shown in FIG. 3. Light C is collected from the chamber environment 230 within chamber 214 with a lens 218, and passed through a bandpass filter 220 and into a photodiode 212. Here, the bandpass filter 220 defines the wavelength of light being used for interference and blocks light at unwanted wavelengths to prevent the plasma background from reaching the photodiode 212. The etching rate is calculated as ##EQU5## where .delta..sub.tm is the measured time between received adjacent (in time) maxima or adjacent minima in the interferometry signal.
Using the known apparatus, the spatial relationship between the region of the film 208 being analyzed, the lens 218 and the photodiode 212 is such that the light from a specific spot on the film is not focused on the photodiode. The photodiode is located roughly at the focal length of the lens. Thus, all of the light from over a relatively large region of the film 208 is delivered to the photodiode, which registers essentially an average effect over the entire area. Even if light from only a relatively small region of the film is focused onto the photodiode, the photodiode signal does not provide any information about the location of the region on the film. Thus, another method must be used to determine this location, such as a laser pointing device, or a line of sight approximation.
Both optical emission and laser interferometry have been used to monitor etch rate at different locations in situ. Davies et al. monitored photoresist thickness using a white light excitation source and a photodiode spectrometer array. Etching rate could have been determined at up to five regions on a wafer; however, this was not done (the method was mentioned, but etching rate measurements were made only over a single region). See John T. Davies, Thomas Metz, Richard N. Savage & Horace Simmons, "Real-time, in situ Measurement of File Thickness and Uniformity During Plasma Ashing of Photoresist," Proc. SPIE, 1392, 551 (1990), using white light and a view window as large as the analysis region.
Economou et al. measured etch rate in situ at five regions across a wafer by multichannel laser interferometry. However, their technique is not readily applicable to industrial plasma etching tools. It requires a-priori selection of sites for analysis and the splitting and alignment of multiple laser beams. Without some extrinsic evidence, such as correlation with an image taken by a standard image capture device, it is difficult to know from the light signal collected what location on the wafer is being analyzed. It is similar to viewing the night sky through a telescope, without being able to take stock of a larger field of view to identify the relationship between the scene in the telescope and the known constellations.
Also, the multichannel laser interferometry technique requires an optical viewport the same diameter as the wafer being measured to obtain a full wafer view. See D. Economou, E. Aydil & G. Barna, "In situ Monitoring of Etching Uniformity in Plasma Reactors," Solid State Technology, 34(4), 107 (1991). Wafers are now normally 200 mm across and will soon be 250-300. Standard viewing windows that are available in reactors are on the order of 50 mm (two inches) across. It is difficult to change the size of a window in a standard reactor. Further, changing the size of such a window may change conditions inside the reactor. In addition, it is advantageous for the etching process to keep the window as small as possible. It is commonly necessary to replace windows that are affected by the etching process. Smaller windows are less expensive to replace. Further, the etchant removes window constituents and those constituents become part of the plasma environment, acting as contaminants. It is beneficial to minimize this contaminating effect.
The use of in situ ellipsometry to measure etching rates is fairly new and has not been used to measure rate uniformity.
It would also be beneficial to be able to determine the uniformity of the rate of film removal in processes other than plasma wafer etching.
In addition to the removal of thin film material, it is useful to be able to know the rate of change of film thickness in processes where additions are made to the thickness of regions of thin films, over the entire surface area of the film. Such processes include: sputter deposition; chemical vapor deposition ("CVD"); plasma enhanced CVD; physical vapor transport; evaporation, thermal processing and others.
It is also useful to be able to determine the absolute thickness of a thin film, either as it is being etched away from or added to a preexisting substrate. For instance, in etching, it is useful to know when all of the film at a certain location has been etched away from a substrate (the so-called process "end-point") so that the etching process can be stopped. End-point is typically identified by observing a characteristic, and then noting when the characteristic undergoes a gross change. The change typically signifies that the endpoint has been reached, and that the process has changed. For instance, as shown in FIG. 1, as the thin film 100 is being etched away, the plasma environment will include a certain chemical composition. After the thin film is all removed, the substrate wafer 120 begins to slowly etch away, and the chemical composition of the plasma environment changes. This change in environment can be noted, and used to identify the occurrence of the endpoint.
This method of endpoint determination has drawbacks. First, the wafer reaches endpoint at different times at different locations around the wafer. However, the known methods typically give only information about the state of a single point or the average state of the entire wafer, for instance when endpoint has been reached at enough locations so that the chemical composition of the reactor environment has changed to a certain degree. Thus, known techniques are not very sensitive. Further, such techniques require that the process actually proceed beyond the minimal endpoint. This is undesirable, for processes which can not tolerate much, if any, overshoot. Further, such techniques are not suitable for monitoring operations where film is being added to a layer, rather than being removed, because there is typically no physical change in the environment when such an accumulative endpoint has been reached.
There are also other instances in which it is beneficial to know the absolute thickness of a film, for instance to evaluate the uniformity of film thickness at the outset or end of a process.
Thus, a significant need exists to be able to monitor and measure the uniformity across a wafer of the rate that material is being etched away from or deposited upon a substrate. It is desirable to be able to monitor etching rate in real-time, as the film material is being etched away, so that process parameters can be changed or observed to bring the etching rate into uniformity or as desired. There is also a similar need for apparatus that can monitor and measure the rate of addition of material to a substrate. It is desirable to monitor the process over the entire surface of a workpiece, such as a wafer, for instance on the order of at least 200 to 300 mm in diameter, while requiring a viewing port of no more than a much smaller diameter, e.g. on the order of 50 mm, as is common with a standard plasma reactor. It is also an object of the invention to determine the absolute thickness of films that are being either etched away from or added to a body of film, without needing to know the original thickness, or the history of etching rate. Further, it is desirable to be able to measure the absolute thickness over the entire surface area of a thin film, through a viewing port that is significantly smaller than the subject film. Another object of the invention is to readily identify the locations on the thin film that are being analyzed, without need to resort to an additional image capture or laser direction device.